The present invention generally relates to drill string flow control valves and more particularly, drill string flow control valves for prevention of u-tubing of fluid flow in drill strings and well drilling systems.
Managed Pressure Drilling (MPD) and Dual Gradient Drilling are oilfield drilling techniques which are becoming more common and creating a need for equipment and technology to make them practical. These drilling techniques often utilize a higher density of drilling mud inside the drill string and a lower density return mud path on the outside of the drill string. Examples of such dual gradient drilling techniques are disclosed in U.S. Pat. No. 7,093,662.
In dual gradient drilling, an undesirable condition called “u-tubing” can result when the mud pumps for a drilling system are stopped. Mud pumps are commonly used to deliver drilling mud into the drill string and to extract return mud from the well bore and a return riser (or risers). In a typical u-tubing scenario, fluid flow inside a drill string may continue to flow, even after the mud pumps have been powered down, until the pressure inside the drill string is balanced with the pressure outside the drill string, e.g. in the well bore and/or a return riser (or risers). This problem is exacerbated in those situations where a heavier density fluid precedes a lighter density fluid in a drill string. In such a scenario, the heavier density fluid, by its own weight, can cause continued flow in the drill string even after the mud pumps have shut off. This u-tubing phenomenon, can result in undesirable well kicks, which can cause damage to a drilling system. For this reason, it is desirable that when mud pumps in a drilling system are turned off, the forward fluid flow be discontinued quickly.